Researchers find new path for nitrogen fixation

Scientists previously regarded phosphorous as the limiting factor for nitrogen fixation because it is an essential component of adenosine tri-phosphate (ATP), the primary source of cellular energy, ecology and evolutionary biology (EEB) professor Lars Hedin said. ATP is needed to power biological reactions, including nitrogen fixation.

“When you have very old and weathered soils, they’re not only very low in molybdenum, [but] very low in phosphorous,” he explained. “The idea of trace metal limitation is not really something that has been formally considered for land systems,” though research from 10-15 years ago proved the importance of trace metals in ocean ecosystems, he said.

Alex Barron GS ’07, the lead author on the paper, said that the researchers’ interest in nitrogen fixation in tropical rainforests prompted them to embark on this project. “Ecologists agree that [nitrogen fixation] is a crucial process and estimate that almost a third of it occurs in tropical forests,” he said in an e-mail, “but almost nothing is known about the factors that control the rate of fixation in these systems.”

The researchers treated land plots in the lowland Panamanian rainforests with various micronutrients. They found that the fixation rate increased in response to treatments of molybdenum alone but not to phosphorous alone.

Barron stressed that this discovery does not undermine the importance of phosphorous in the nitrogen fixation process. “Phosphorus is still likely to control rates of nitrogen fixation in many systems,” he explained. “However, our work shows that ecologists should also be checking for molybdenum. And, since molybdenum is found naturally in phosphorus fertilizer, our work also shows scientists need to be very careful not to conclude [that] phosphorous is the limiting factor when it might be the molybdenum ‘contamination’ causing the response,” he added.

Used today in manufacturing for its good electrical conductivity and anticorrosive properties, molybdenum is a cofactor for nitrogenase, the enzyme that carries out nitrogen fixation. Nitrogenase is a catalyst in the reaction that converts nitrogen into ammonia.

“There have been other demonstrations of molybdenum limitation in ... forest systems, but for tropical forests, this was the first documentation that molybdenum had a stronger impact than did phosphorous,” said Nina Wurzburger, a member of the research team and a research associate in the EEB department. “That was the new and exciting result of this work.”

In the ecosystem studied by the researchers, molybdenum is roughly 10,000 times scarcer than phosphorous.

Hedin noted that molybdenum limitation is most easily observed “in the tropics and only in certain tropical forests ... that are very old and where molybdenum has weathered out from the soil.” Hedin added that molybdenum limitation may also be a factor in areas with poorly managed agricultural systems in Australia and some forests in the Pacific Northwest of the United States, which experience high rainfall.

The researchers said they believe that their research has possible implications for understanding the tropical rainforest ecosystem’s response to climate changes caused by carbon dioxide. The tropical rainforests are a major sink for carbon dioxide, the researchers said, and the continued uptake of the greenhouse gas is dependent on the amount of nitrogen.

“If molybdenum influences the nitrogen cycle, it can also influence the productivity of tropical forests, and this has an implication for the global carbon cycle,” Wurzburger said.

“Like a factory that runs out of one crucial widget, or a runner who ‘hits the wall’ because she runs out of glycogen, the uptake of [carbon dioxide] by some ecosystems could fall well short of what models predict” if molybdenum limits nitrogen fixation, Barron explained. “This means a world that warms even more rapidly.”

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A second implication is the possibility of supplementing the soil with molybdenum — for tasks like “replant[ing] a tropical forest or increas[ing] productivity in a coffee plantation” — to increase the uptake of atmospheric nitrogen, Barron said.

The researchers are continuing their work to determine the extent of molybdenum’s role in controlling nitrogen fixation. “As with any good study, this work raises more questions than it answers,” Barron said.

Other contributing authors include Anne Morel-Kraepiel, an associate research scholar in the chemistry department, Jean-Philippe Bellenger, an associate research scholar at the Princeton Environmental Institute, and S. Joseph Wright ’74, a staff scientist at the Smithsonian Tropical Institute in Panama.